EFFECT OF SOIL TEMPERATURE ON LONGIDORUS AFRICANUS DAMAGE TO CARROT AND LETTUCE SEEDLINGS Xiang Huang and Antoon T. Ploeg Department of Nematology, University of California Riverside, CA 92521, U.S.A. ABSTRACT Huang, X., and A. T. Ploeg. 2001. Effect of soil temperature on Longidorus africanus damage to carrot and lettuce seedlings. Nematropica 31:87-93. The effect of soil temperature on Longidorus africanus damage to lettuce and carrot seedlings was studied in greenhouse experiments. Cones containing 70 ml sterilized sand were seeded with carrot or lettuce, inoculated with a range of L. africanus densities and placed in waterbaths maintaining soil temperatures of 17, 20, 25, or 30°C. At harvest, 30 days and six weeks after inoculation for lettuce and carrot respectively, lettuce dry weights and carrot fresh weights were determined. Data analysis showed that minimum yields were significantly lower at 25°C than at 17°C but that effects of soil tem- perature on tolerance limits were very minor. Results from this study suggest that seeding of carrot or lettuce at soil temperatures ≤17°C would significantly reduce damage by L. africanus. Key words: carrot, damage function, lettuce, Longidorus africanus, minimum yield, temperature, tolerance. RESUMEN Huang, X., and A. T. Ploeg. 2001. Efecto de la temperatura del suelo sobre el daño causado po Lon- gidorus africanus en plantas de zanahoria y lechuga. Nematrópica 31:87-93. El efecto de la temperatura del suelo sobre el daño causado por Longidorus africanus en plantas de lechuga y zanahoria se estudió en condiciones de invernadero. Semillas de zanahoria o lechuga pre- viamente incubadas con diferentes densidades de L. africanus se sembraron en conos que contenían 70 ml de arena esterelizada. Posteriormente, los conos se colocaron en baño maría para matener las temperaturas del suelo a 17, 20, 25, ó 30°C. La cosecha se realizó a los 30 días y seis semanas después de la inoculación de la lechuga y zanahoria respectivamente. En ese momento se midió el peso seco de la lechuga y peso fresco de la zanahoria. El análisis de los datos demostró que los mínimos rendi- mientos fueron significativamente más bajo a 25°C que a 17°C, pero el efecto de la temperatura del suelo en los limites de tolerancia fueron menores. Estos resultados sugieren que el cultivo de la le- chuga y zanahoria en suelos cuya temperatura es inferior a los 17°C reduciría significativamente el daño causado por L. africanus. Palabras claves: función de daño, lechuga, Longidorus africanus, redimiento mínimo, temperatura, tol- erancia, zanahoria. INTRODUCTION Heyns, 1987; Radewald et al., 1969a; Zeidan and Coomans, 1992). This nema- The distribution of the ectoparasitic tode has a wide host range (Cohn and Mor- nematode Longidorus africanus Merny dechai, 1969; Kolodge et al., 1986, 1987), appears to be restricted to areas with hot but in the Imperial Valley of Southern Cal- desert-type climates e.g., Egypt, Israel, ifornia, USA, carrot and lettuce are partic- Portugal, Sudan, South Africa, and South- ularly susceptible to damage (Lamberti, ern California (Aboul-Eid, 1970; Bravo 1986; Radewald et al., 1969a). Above- and Roca, 1995; Cohn, 1969; Jacobs and ground symptoms of L. africanus infesta- 87 88 NEMATROPICA Vol. 31, No. 1, 2001 tions in these crops are generally non- and restrictions on their use, alternative specific and may include stunting, chlorosis methods need to be developed. Adjust- and wilting (Kolodge et al., 1986; Radewald ment of sowing or planting dates to peri- et al., 1969a). In lettuce, L. africanus causes ods when nematode population levels are root-tip galling and severe retardation of generally low or nematodes are inactive, root growth (Huang and Ploeg, 2001; has been shown to be promising for reduc- Radewald et al., 1969a). Typical symptoms ing crop damage by other nematodes in carrot include stubbing and forking of (Belair, 1987; Jeffers and Roberts, 1993; the taproot (Huang and Ploeg, 2001; Pacumbaba and Tadesse, 1991; Roberts, Kolodge et al., 1986), that may result in 1987). In the Imperial Valley, CA, where severe economic losses. carrot and lettuce are sown nearly year Relationships between initial nematode round, average soil temperatures at 15 cm population densities (Pi) and plant growth depth range from ca. 30°C in summer, to can be modeled according to Seinhorst’s ca. 13°C in winter (Fig. 1). The objective of damage function: this study was to determine the effect of soil temperature on the relationship between initial population densities of L. africanus y/y = 1 for Pi ≤ T, and max and damage to lettuce and carrot seed- y/y = m + (1-m) × 0.95(Pi/T)-1 for Pi > T (I) max lings, and from the results obtained, to evaluate the possibility to avoid crop dam- with ymax representing plant growth in the age by adjusting sowing dates. absence of nematodes, Pi the initial nema- tode density, m the minimum yield MATERIAL AND METHODS obtained even at very high initial popula- tion densities, and T the tolerance limit Longidorus africanus, originally obtained i.e., the lowest population density affecting from carrot field soil in the Imperial Val- plant growth (Seinhorst, 1998). Estimated ley, were cultured on tomato (Lycopersicon values of m and T for L. africanus on carrot esculentum Mill. cv. Pixie) in 1-L pots in a and lettuce were very low compared to sev- greenhouse at a constant soil temperature eral other nematode-crop associations, of 26°C in steam-sterilized coarse sand. indicating the high susceptibility of these Subculturing took place every 2 to 3 crops to damage by this nematode species (Huang and Ploeg, 2001). Field studies in the Imperial Valley, CA, showed a strong correlation between the ver- tical distribution of L.africanus and soil tem- perature, with high populations occurring in the upper soil layers during the hot summer months (Ploeg, 1998). The relatively high optimum soil temperature of ca. 30°C for L. africanus multiplication (Lamberti, 1969; Ploeg, 1999) confirmed the preference of this nematode for warm conditions. Longidorus africanus can be effectively Fig. 1. Monthly average soil temperature (°C) at 15 cm depth for El Centro, Imperial County, CA over the controlled with nematicides (Radewald et 1989-2000 period. Calculated from data at http:// al., 1969b), but because of increasing costs www.ipm.ucdavis.edu/. Dashed line at 17°C. Soil temperature and L. africanus: Huang, Ploeg 89 months. Nematodes used as inoculum and the root systems were washed free of were extracted from the cultures with a soil. Tops were separated from the roots modified sieving and decanting technique and placed in an oven at 65°C. Four days (Brown and Boag, 1988). Final separation later, dry weights of tops and roots were was by migration of the nematodes determined. An experiment using carrot through a 100 µm pore nylon sieve for 12 cv. Triumph (Petoseed, Saticoy, CA, USA) hours into a plastic saucer filled with was set-up in a similar manner, but with enough water to touch the bottom of the inoculum densities of 0, 5, 10, 20, 50, 100 sieve. Inocula containing 5, 10, and 20 and 200 L. africanus added per cone (70 L. africanus per 2 ml were handpicked ml soil) resulting in a total of 168 cones (7 from the suspension using a fine needle, inoculum densities × 4 temperatures × 6 higher inoculum densities were prepared replicates). Carrots were harvested six by concentration or dilution of the origi- weeks after inoculation and tap root fresh nal nematode suspension. weight and length was determined. Rela- Three seeds of lettuce (Lactuca sativa, tionships between plant growth and L. afri- L. cv. Burpee’s Iceberg, Burpee Seeds, canus inoculum densities were fitted to the Warminster, PA) were sown in each of 192 Seinhorst model (I), and LSD’s between plastic cones (80 ml volume; Stuewe & estimated parameters were calculated Sons, Corvallis, OR), closed at the bottom using SAS statistical software (SAS Insti- and filled with 70 ml steam-sterilized sandy tute, Cary, NC, U.S.A.). soil. The cones were placed in 48, 1-L plas- tic cups (4 cones per cup), and the space RESULTS between the cones and the inside of the cup was filled with sand. The cups were Longidorus africanus caused obvious then placed in waterbaths running at 17, symptoms on lettuce and carrot consisting 20, 25 or 30°C (±1°C), with the water level of stunted top and root growth, root tip ca. 1 cm below the upper rim of the cup, galling (lettuce and carrot) and tap root but level with the upper rims of the cones forking (carrot). Growth of the “no nema- in the cups. Thus, each waterbath con- tode control” carrot and lettuce plants was tained 12 cups holding a total of 48 cones. affected by temperature, with the carrots One of eight L. africanus inoculum levels grown at 30°C, and the lettuce grown at (0, 5, 10, 20, 50, 100, 200, or 400 nema- 17°C resulting in the smallest plants todes) was assigned at random to each of (Table 1). When fitting the data to the these 48 cones (six cones per inoculum Seinhorst model (I), more than 95% of level) for each waterbath. Immediately the observed variance was accounted for, after seeding, a shallow hole was made in indicating a good fit (Figs. 2, 3, and 4). the sand in each cone and 2 ml of a sus- Tolerance limits for lettuce top growth pension containing the previously assigned (dry weight) were not significantly differ- number of L. africanus were poured into ent (P > 0.05) between the four soil tem- the hole in each cone. Soon after emer- peratures and averaged at ca.